Technical Talk--Can Speaker Performance be Measured

In three words, yes and no. Now let me expand on that oversimplified answer.
Many aspects of a loudspeaker’s performance can be measured, but we are concerned,
first and foremost, with somehow de fining the sound quality of a speaker system
(normally, a pair of speakers positioned somewhat arbitrarily in an undefined
room). This, after all, is a major goal of the speaker reviews in STEREO REVIEW
and other audio publications.

Let us approach this problem from two directions: (1) the acoustical stimulus
of the listener’s eardrums by the sound field created in a room by the speakers,
and (2) the electrical signal presented to the speakers’ input terminals
by an amplifier.

The characteristics of the input signal are (or can be) known with considerable
accuracy and can be taken as the reference to which the acoustical stimulus
is compared in order to establish how faithfully the input signal has been
translated into an acoustic pres sure wave entering the listener’s ear. One
might assume that if the two waveforms (acoustic and electrical) are identical,
then the transfer of the electrical input signal to the listener’s brain
(or at least to one of its entrance ports) has been accomplished with a reasonable
degree of accuracy. Alas, things are not that simple.

For one thing, in order to measure the sound pressure at the eardrum, it
is necessary to insert a tiny probe micro phone into the subject’s ear. This
is hardly a normal listening condition, although the procedure is used in
psychoacoustic research (usually to mea sure the subject’s hearing acuity
rather than the quality of a loudspeaker).

The usual alternative way of dealing with this matter is to use a calibrated
measurement microphone placed approximately where a listener’s ear might
be located in an actual hi-fi installation. Since reflections of the sound
from the boundaries of the room, as well as the standing waves created in
the room by those reflected signals, can have a profound effect on the result
of such a measurement, it is often made in an anechoic chamber (a room designed
to absorb sound on all its inner surfaces, thereby eliminating or reducing
reflected energy).

There are several problems in the use of an anechoic chamber for measurement
of loudspeaker sound quality, although it can be invaluable for determining
the characteristics of individual drivers. In order to remain echo-free down
to low-bass (or even mid-bass) frequencies, the chamber must be very large,
and consequently very expensive. Although many loud speaker manufacturers
have anechoic chambers, few of these are big enough for accurate measurement
of the bass output of even a moderate-price speaker. Fortunately, there are
other methods of realizing many of the benefits of anechoic measurement without
the use of an anechoic chamber. In general, these involve a mathematical
computation process known as the fast Fourier transform (FFT).

There is a more serious difficulty in the use of anechoic test chambers
for evaluating speaker sound quality, as opposed to the frequency response
of specific drivers or combinations of drivers. The problem is that we do
not listen to high-fidelity music systems in anechoic chambers. The sound
is, to put it mildly, unsatisfactory. The acoustic properties of the listening
room can have as much to do with sound quality as the actual performance
of the speakers themselves.

Although this linkage of room and loudspeaker performance makes the prediction
of a speaker’s sound in any particular room rather uncertain, there are still
ways to make measurements in an ordinary room that can be useful for a rough
overall assessment of a speaker’s capabilities. And these methods are much
less expensive (and more economical of space) than building an anechoic chamber.

Unfortunately, there are no meaningful test standards that enable measurements
made by different people to be interpreted by others to predict the sound
quality of a speaker. It is difficult enough for a tester to do that from
his own measurements, let alone from someone else's.

Next month, I will describe in some detail how I test speakers and interpret
those measurements, but I would like to close this column by commenting on
a somewhat related problem posed by a reader, who wonders about the effect
of the nonlinear frequency sensitivity of the human ear on loudspeaker com
parison tests. He postulates two speakers, one (A) with a rising low-frequency
response and the other (B) with a “flat” response. (Let’s not worry at this
point about what a “flat” response from a speaker might be.)

Aware that a speaker comparison must be made at identical listening levels
(also not as easy to achieve as some people think), this reader suspects
that at low levels speaker. A would sound relatively flat, since its inherent
response resembles a loudness-compensation curve. (Such a response curve,
as supplied by the loudness control of an amplifier or receiver, is intended
to compensate for the human loss of sensitivity to low frequencies at reduced
listening levels.) On the other hand, speaker B would sound deficient in
lows because of the same effect. At a higher listening level, how ever, the
emphasized lows of speaker.

A would be audibly excessive, where as speaker B would presumably sound
as “flat” as it measures. By extension, a speaker deficient in lows should
sound more accurate at high volume levels.

The reader’s question is, “What is the correct listening level at which
to compare speakers?” He suspects that, if comparisons are done in a dealer’s
listening room, they should be done at levels similar to those one would
use at home.

I think he is correct, although I must confess I have never given the matter
much thought. I never give too much weight to any speaker demonstration I
hear outside my own home. I have rarely found any speaker that sounded the
same at a manufacturer’s or dealer’s demo as it does in my own listening
room (it might be better, worse, or simply different). This may result in
part from the relative volume-level differences, but I suspect it is largely
a function of room acoustics or the specific locations of the speakers. A
manufacturer’s demo, if it is done correctly, is usually preceded by careful
positioning and orientation of the speakers, along with acoustic treatment
of the room. The moral is, don’t buy any speakers unless the dealer will
permit you to return them if they don’t sound satisfactory in your own listening
room.